1. Field of the Invention
The present invention relates to a semiconductor photonic device, a method for making the same, and a method for forming a ZnO film. In particular, the present invention relates to a semiconductor photonic device using a Group III-V compound, such as GaN, InGaN, GaAlN or InGaAlN and a method for making the same. Also, the present invention relates to a method for making a ZnO film formed on a substrate, such as a Si substrate or a glass substrate.
2. Description of the Related Art
As materials for semiconductor photonic devices, such as light-emitting diodes (LEDs) and semiconductor laser diodes (LDs) which emit blue or ultraviolet light, Group III-V semiconductor compounds represented by the general formula In.sub.x Ga.sub.y Al.sub.z N wherein x+y+z=1, 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, and 0.ltoreq.z.ltoreq.1 are known. Since the semiconductor compounds are of direct transition type, they have high luminescent efficiency. Furthermore, since the luminescent wavelength can be controlled by the indium content, they have attracted attention as materials for photonic devices.
Since it is difficult to make a large In.sub.x Ga.sub.y Al.sub.z N single crystal, a so-called heteroepitaxial growth process in which an In.sub.x Ga.sub.y Al.sub.z N film is grown on a substrate composed of a different material is used in the production of the crystal film, and it is typically grown on a C-plane sapphire substrate. Because the C-plane sapphire substrate is expensive and there exists a large lattice mismatch between the C-plane sapphire substrate and the In.sub.x Ga.sub.y Al.sub.z N film (for example, the lattice mismatch rate for GaN ranges to 16.1%), many crystal defects with a defect density of 10.sup.8 /cm.sup.2 to 10.sup.11 /cm.sup.2 are inevitably formed in the grown crystal, and thus a high-quality crystal film having high crystallinity cannot be formed.
In order to solve this problem, a proposed method for obtaining a crystal with decreased defects by reducing lattice mismatch when In.sub.x Ga.sub.y Al.sub.z N is deposited on a C-plane sapphire substrate is to provide a polycrystalline or amorphous AlN buffer layer or a low-temperature-deposited GaN buffer layer on the C-plane sapphire substrate. Since this method can reduce the lattice mismatch not only between the C-plane sapphire substrate and the buffer layer but also between the buffer layer and the In.sub.x Ga.sub.y Al.sub.z N, a crystal film with reduced defects can be formed. The C-plane sapphire substrate used in this method, however, is expensive and since the configuration is complicated, higher production costs are unavoidable.
A SiC substrate has been studied and has small lattice mismatch (for example, the lattice mismatch rate for GaN is 3.5%). The SiC substrate, however, is considerably expensive compared to the C-plane sapphire substrate (its price is approximately ten times the price of the C-plane sapphire substrate).
Accordingly, production of a semiconductor photonic device using an inexpensive Si or glass substrate has been desired. A possible method is depositing a ZnO buffer layer on a Si or glass substrate, and providing a GaN layer on the ZnO buffer layer followed by forming an In.sub.x Ga.sub.y Al.sub.z N semiconductor layer for emitting light on the GaN layer (or providing an In.sub.x Ga.sub.y Al.sub.z N semiconductor layer containing a GaN layer). Because the lattice constant in the a-axis direction (hereinafter referred to as "a-constant") and the lattice constant in the c-axis direction (herein after referred to as "c-constant") of the ZnO single crystal are nearly equal to the a-constant and the c-constant, respectively, of GaN, a GaN layer with reduced lattice defects is considered to be formed. The ZnO crystal is hexagonal and the crystal grows so that the c-axis direction is perpendicular to the surface of the Si or glass substrate whereas the a-axis direction is parallel to the surface of the Si or glass substrate.
TABLE 1 Crystal a-constant c-constant GaN 3.1860 .ANG. 5.1780 .ANG. ZnO 3.24982 .ANG. 5.20661 .ANG.
A device having a ZnO buffer layer provided on a Si substrate has a substrate cost which is approximately one-tenth that of a C-plane sapphire substrate and thus cost reduction can be achieved. Since the Si substrate can have conductivity in contrast to insulating characteristics of the C-plane sapphire, a p-type electrode and a n-type electrode can be provided on the upper face and the lower face and the device configuration can be simplified.
A lattice mismatch rate of 2% is still present between the ZnO buffer layer formed on the Si substrate and GaN layer, as shown in Table 1, although the rate is smaller than that in a combination of GaN and a C-plane sapphire substrate or a SiC substrate. Thus, defects formed by the lattice mismatch still remain.